1. Field of the Invention
The invention relates to a combination of cellulolytic enzymes with cationic and anionic polymers for use in enhancing the freeness of clarified sludge in papermaking.
2. Description of the Prior Art
More and more the papermaking industry uses recycled papers. For example, for the manufacture of corrugated cardboard, raw materials which are based on recycled fibers are being used more frequently and, at the same time, the number of recyclings is increased. With each recycling, the quality of the raw materials is lessened so that fiber strength is reduced, and more fines are generated. Further, more contaminants are accumulated. All of these problems result in decreasing freeness of pulp.
One product of papermaking is so-called clarified sludge. Clarified sludge is a concentrate of pulp and paper mill effluent which contains solids primarily in the form of fiber fines. Old newsprint (ONP) is another type of recycled furnish. The fiber fines found in clarified sludge usually are smaller than 10 microns. Also contained within clarified sludge are hemicellulose and chemical substances such starch, rosin, alum, hot melts (commonly referred to as stickies and tackies) and organic matter. Clarified sludge is typically land filled at a tremendous cost to the paper makers and the environment. The amount and types of fines (commonly referred to as "anionic trash") are too difficult and uneconomical to treat by conventional mechanical/chemical methods. These methods include refining, screening and treatment with retention and drainage aids. In the industry, so-called recycle furnish, which is an aqueous suspension that has gone through the papermaking process 1 to 2 times, is used.
ONP should also be distinguished from other recycle pulps including old corrugated containers (OCC) which are treated in a pulper with hot water under a continuous agitation until a pulp is produced. Typically, OCC fibers have a greater length then clarified sludge, since they are a mixture of chemical mechanical pulp (CMP) and chemical thermal mechanical pulp (CTMP) which are derived from hardwoods and kraft cuttings. Further, clarified sludge differs from OCC in many physical characteristics, including consistency, bulk viscosity, pH, charge, fiber strength and the composition of solid contents. The clarified sludge contains abundant viscous microbial polysaccharides. These biopolymers hold copious amounts of water and are difficult to treat with conventional methods. Enzymes can break down the polysaccharide structure which may enhance the drainage of the sludge.
The pulps in aqueous suspension which are ready to be used on a paper machine can be characterized by various parameters, one of which is particularly significant for predicting the draining capability of the pulp. A measure of the drainability of the pulp is frequently expressed in the term "freeness". Specifically, freeness is measured according to Canadian Standard Freeness, or CSF measurement. CSF measures the drainage of 3 grams (oven dried weight) of pulp suspended in one liter of water.
Use of cellulolytic enzymes, e.g. the cellulases and/or the hemicellulases for treating recycled paper pulps to improve freeness is the subject of U.S. Pat. No. 4,923,565 the disclosure of which is incorporated herein by reference. The cellulase enzyme described in the '565 patent may be used in the practice of the present invention.
U.S. Pat. No. 5,169,497, issued to Sarkar and Cosper discussed the effects of cellulases in combination with cationic flocculants of varying composition on the freeness of old corrugated containers (OCC) pulp. The '497 patent covers the use of a combination of enzyme and cationic polymers for enhancing the freeness of recycled fiber. In practice, dual polymer treatment programs are also used for retention.
The pulp is first treated with a cellulolytic enzyme followed by cationic and anionic polymers. In a dual polymer retention system, two synthetic polymers are mixed with the pulp sequentially to achieve better results than obtained with either polymer by itself. Usually, a low molecular weight, highly charged cationic polymer is added to the papermaking furnish first, and then at a later stage, a high molecular weight, anionic polymer is added. Dual polymers have found a place in paper and board manufacturing. Good retention has numerous economic benefits. As the use of recycled fiber increases in container board, fine paper, and newsprint grades, the opportunity to provide benefits through retention aids has also increased. If fines are not retained by a good retention aid or hydrolyzed by an enzyme, they will impede drainage, fill felts, and cause deposition problems. The key benefit of retention aids with enzyme is to prevent drainage reduction and subsequent loss of machine speed. Drainage can be maintained by preventing the build-up of fines in the white water loop.
U.S. Pat. No. 5,308,449, issued to Fuentes et al. discusses the use of enzymes as a method of treating recycled paper for use as a papermaking pulp. Fuentes et al. does not addresses the problem of clarified sludge in the recovery of pulp from within that sludge for later use in papermaking. Further, there is no discussion in Fuentes et al. of the use of treatment agents for enhancing the freeness and drainability of pulp once the recycled paper has been introduced back into the papermaking process.
Ideally, a method would exist which would allow for the recovery of paper pulp from clarified sludge while at the same time increasing the freeness and drainability of any resulting paper pulp once it is processed through the papermaking machinery.
The object of the present invention to disclose a method of treating previously unused clarified sludge for re-use in the papermaking system. By re-using clarified sludge, substantial economic benefits may be derived in terms of decreased waste removal cost as well as increased efficiency in the use of materials by the papermaking industry. Since old newsprint pulp is significantly less costly than OCC pulp, papermaking mills will recognize significant economic benefits.